tcp: Add hash histogram output and validate bucket length accounting

Provide a histogram output to check, if the hashsize or
bucketlimit could be optimized. Also add some basic sanity
checks around the accounting of the hash utilization.

MFC after: 2 weeks
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29506